The significance of β-carotene, α-tocopherol and the xanthophyll cycle in droughted Melissa officinalis plants

Diurnal variations in pigment composition, α-tocopherol and photosynthesis were measured during the summer drought and recovery after autumn rainfalls in Melissa officinalis L. grown under Mediterranean climate. Summer drought caused a drastic decrease in relative water content and water potential of leaves to ca 35% and −3 MPa respectively, indicative of severe stress. Chlorophyll levels decreased progressively in drought, associated with the inability to increase total carotenoids, and especially ß-carotene. In contrast, α-tocopherol increased by ca 60% in response to drought and showed a significant decrease at midday in severely stressed plants. The intrinsic efficiency of PSII photochemistry remained unchanged at predawn throughout the experiment and decreased up to ca 65% in plants exposed to the interaction of high light and drought, associated with an enhanced de-epoxidation state of the xanthophyll cycle. Although droughted M. officinalis plants showed a significant decrease in the endogenous levels of photosynthetic pigments, the antioxidant function of a-tocopherol and the dissipation of excess excitation energy by the xanthophyll cycle may help to avoid irreversible damage to the photosynthetic apparatus.